As a rule, if you won't be performing numeric calculations on the data, entries should be stored as text. Social Security numbers, Phone numbers and postal codes should be stored as text.

You can use alphabetic characters in an input mask. For example, one of the sample input masks is >L0L\ 0L0 used to represent a Canadian postal code.

The ">" character in the input mask converts all the characters that follow to uppercase.

The "L" character requires an alpha entry; the "0" (zero) requires a numeric entry.

A "\"character causes the following character to be displayed as a literal character rather than a mask character.

A space appears between the three character pairs.
For example, V5P 2G1 is one valid postal code that the user could enter. The mask would prevent the user from entering two sequential alphabetic characters or numbers.
See:

You can manipulate postal codes in Access by changing the data type, input mask, or format of a postal code field.